Various organs and passages in the body are subject to the development of stones, calculi and the like. Gallstones are a common problem in the United States and are the most frequent cause of gallbladder inflammation. Calculi in other parts of the biliary system are also commonplace. Similarly, stones, calculi and the like can develop throughout the renal or urinary system, not only in the ureters and distal to them, but also in the renal tubules and in the major and minor renal calyxes. The calyxes are hollow collecting structures in the kidneys, extending from the renal pelvis, the minor calyxes in particular joining the renal pyramids. For simplicity, the calyxes can be considered as ducts extending from the connecting tubules of the renal nephrons to the ureters.
Minimally invasive surgical procedures have been developed for the removal of stones, calculi and the like from the biliary and urinary systems. Such procedures avoid the performance of invasive, open surgical procedures (such as, for example, the cholecystectomy) and can instead employ percutaneous access, in which stones, calculi and the like are removed through a percutaneously inserted access sheath. Several access routes are suitable, depending upon the specific system and the particular location in the system at which the stones, calculi or the like are found. Without regard to the access route, however, percutaneous extraction is usually based upon the use of either forceps or basket-tipped catheters to engage and remove the stones, calculi and the like.
While forceps enable the quick extraction of stones positioned within ready reach, they are best used in conjunction with endoscopes, to avoid the inadvertent penetration, damage or extraction of intact mucosa. Forceps usually are not suited for the extraction of stones, calculi or the like which are not positioned relatively close to the exits of the biliary and urinary tracts, since forceps cannot negotiate multiple curves in the tracts. It is true that these drawbacks can be partially overcome by the use of basket-tipped catheters which can traverse such winding courses. However, such catheters are often unable to seize impacted stones.
More particularly, basket-tipped catheters can be less than satisfactory in attempting to retrieve very small or flat stones, particularly when the stones lie in large cavities which provide the stones with ample room for displacement. Moreover, most known stone extraction baskets are helically shaped, so that they permit entry of the stone only from the side of the basket. This is typically due to the presence of a discreet tip on the basket, such as a small length of cannula which holds the ends of the wires of the basket together. Thus, a head-on or open-ended approach to the stones is not possible with these "leading tip" retrieval baskets. Moreover, such helically-shaped baskets have a tendency to fold on themselves, rather than to open, when pushed against a wall of a hollow organ. This folding reduces the chance of capturing stones. In addition, the relatively sharp tip of such baskets tends to cause an indentation in the organ wall during capture of the stones, and can lead to damage or injury to the wall of the organ.
There are "open-ended" baskets available in which the wires of the basket form an open loop, and thereby permit a head-on approach to the stones, calculi or the like. The wires of such baskets are formed as plural loops arranged in a manner similar to the loops of a common kitchen tool, the eggwhip, and for that reason baskets of this type are sometimes referred to as eggwhip-type baskets. The wires of such baskets have typically been composed of stainless steel. Stainless steel, however, is often subject to undesirable kinking during use, making it very difficult to push or track the device through the body. Moreover, stainless steel baskets typically do not have a resiliency suitable for more than one stone capture. These drawbacks become particularly acute as the diameter of the device is decreased, as is required for the retrieval of stones located more deeply within the body.
A few devices are known in which the wires of the basket are composed of a superelastic alloy (for example, nitinol), but these particular devices may be subject to their own drawbacks in manufacture or use. For example, they commonly employ suture material to interconnect the most distal ends of the loops to one another. A coating material or adhesive is often applied to the suture material and the underlying section of wire, to fixedly position the suture material and wires relative to one another and prevent the basket loops from sliding at their interconnection. In the absence of such a material or adhesive, the sliding of the loops is allowed to be fairly free, even in the presence of the suture material. No provision is made in these devices for an intermediate degree of restricted sliding or movement of the basket wires with respect to one another. Nor is any provision made, in devices having relatively fixed basket wires, for allowing substantial flexing of the individual wires with respect to one another, without unacceptable kinking. Moreover, the construction of retrieval devices of very small diameter, for example, smaller than about 8 French (8 Fr, or about 2.7 mm), can be problematic, particularly in trying to achieve some sort of securement of the distal ends of the loops to one another.
It should thus be clear that it would be highly desirable to have a minimally invasive device for repeatedly grasping, capturing, retrieving, extracting or removing stones, calculi or the like which was tipless and which was atraumatic during use. It would also be highly desirable to have such a device which possessed superior resistance to kinking during use, and which possessed good pushability and trackability during use. It would further be highly desirable to have such a device which possessed a predetermined, restricted degree of relative movement between the wire loops making up the basket, yet which permitted a very high degree of springiness or flexibility between the pair of wire legs making up each wire loop of the basket. It would further be desirable to eliminate the need for a separate or disparate material to limit relative movement of the wire loops. It would also be highly desirable to have such a device which could possess either an even number or an odd number of wire legs making up the basket. Finally, it would be especially desirable to have such a device in unusually small diameters, from below about 8 Fr (2.7 mm) to even below about 2 Fr (0.7 mm).